The invention relates to electric circuits comprising a bandgap reference circuit and a start-up circuit and to methods for starting up a bandgap reference circuit.
Bandgap reference circuits are, for instance, required as voltage or current reference sources in integrated circuits, and normally need a start-up circuit in order to work reliably. Otherwise there may be the risk that the bandgap reference circuits may work at an incorrect operating point. Bandgap reference circuits, for instance, are disclosed in published German application for patent No. 10 2004 004 305 A1.
The principle of bandgap reference circuits is the following: The voltage difference between two diodes is used to generate a proportional to absolute temperature (PTAT) current in a first resistor. This current is used to generate a voltage across a second resistor. The voltage across the second resistor is added to the voltage of one of the two diodes of the bandgap reference circuit or to a further diode.
FIG. 4 shows an example of a bandgap reference circuit 1 and a conventional start-up circuit 42. This example is provided as an illustration of the general problems associated with start-up circuits for bandgap reference circuits.
In this example, the bandgap reference circuit 1 comprises an operational amplifier A1 having an inverting input 3, a non-inverting input 4 and an output 5. The operational amplifier A1 in this example is not an ideal operational amplifier but what is known as an OTA. An OTA is a voltage-controlled current source. The output 5 of the operational amplifier A1 supplies a voltage that is applied to the gate terminals of a first PMOS transistor P1 and a second PMOS transistor P2 to form a closed control loop. A supply voltage VDD is applied to the PMOS transistors P1, P2.
The first PMOS transistor P1 is connected to the inverting input 3 of the operational amplifier A1, to a first diode D1 and to a first resistor R1. The terminals of the first diode D1 and of the first resistor R1 that are on the opposite side from the first PMOS transistor P1 are connected to ground. The node resulting from the connection of the first PMOS transistor P1 to the first resistor R1 and the first diode D1 is denoted by B1.
The second PMOS transistor P2 is connected to the non-inverting input 4 of the operational amplifier A1, to a second resistor R2 and to a third resistor R3. The terminal of the third resistor R3 on the opposite side from the second PMOS transistor P2 is connected to a first terminal of a second diode D2, whose second terminal is connected to ground. The terminal of the second resistor R2 on the opposite side from the second PMOS transistor P2 is also connected to ground. The node resulting from the connection of the second PMOS transistor P2 to the second and third resistors R2, R3 is denoted by B2.
The bandgap reference circuit 1 also comprises an output transistor P3, to which the output voltage Vout of the bandgap reference circuit 1 is applied at an output node BGout of the bandgap reference circuit 1 and across an output resistor Rout connected to ground and the output node BGout.
The gate terminal of the output transistor P3 is also connected to the gate terminals of the two PMOS transistors P1, P2. This connection forms a node BIAP.
In many cases, the operational amplifier A1 draws its bias current from the bandgap reference circuit 1 itself, for instance, by means of an additional current mirror, so that the operational amplifier A1 is also not fully functional until the bandgap reference circuit 1 has started up. The required bias current can also be generated independently of the bandgap reference circuit 1, and has a reasonably well known magnitude.
The conventional start-up circuit 42 for the bandgap reference circuit 1, provided for illustrating the general problem, comprises a PMOS transistor P4, a first NMOS transistor N1 and a second NMOS transistor N2.
The conventional start-up circuit 42 for the bandgap reference circuit 1 works as follows.
If the output voltage Vout of the bandgap reference circuit 1 has not yet reached a certain level, i.e., the bandgap reference circuit 1 has not yet started up, then an auxiliary circuit comprising the PMOS transistor P4 of the start-up circuit 42 and the first NMOS transistor N1 switches on the second NMOS transistor N2. The second NMOS transistor N2 pulls the node BIAP downwards so that an electric current begins to flow in the heart of the bandgap circuit, i.e., inside the bandgap reference circuit 1. The operational amplifier A1 should then assume full control of the bandgap reference circuit 1 at this point in time. Without this start-up assistance, the two inputs 3, 4 of the operational amplifier A1 could sit at ground potential, and the operational amplifier A1 would have no reason to change its state.
If there is sufficient electrical current flow in the heart or core of the bandgap circuit and hence the output voltage Vout at the output node BGout of the bandgap reference circuit 1 is sufficiently high, then the second NMOS transistor N2 of the start-up circuit 42 can be turned off again, so that the bandgap reference circuit 1 is brought automatically into its correct operating point by the operational amplifier A1.
Assuming that the operational amplifier A1 has a non-negligible offset voltage in the negative direction, i.e., the non-inverting input 4 of the operational amplifier A1 must be taken in the negative direction in order to bring its output 5 into the center position, and assuming that the start-up circuit 42 is just being operated at a preliminary operating point, then a “moderate” electrical current flows in the bandgap reference circuit 1. Then the two inputs 3, 4 of the operational amplifier A1 are also taken to a “moderate” start-up state. In this case, it may happen that the general conditions are inadequate for sensible operation of the operational amplifier A1.
If, nonetheless, sensible operation of the operational amplifier A1 of the bandgap reference circuit 1 is possible, then it is conceivable that the operational amplifier A1 is controlling in the wrong direction: if the electrical current within the bandgap reference circuit 1 is not large enough, and hence the electrical voltages across the resistors R1, R2, R3 are not large enough for a non-negligible electrical current to flow through the two diodes D1, D2, then the inputs 3, 4 of the operational amplifier A1 are also driven at a negligible level. Assuming the aforementioned offset voltage of the operational amplifier A1, the operational amplifier then controls in the wrong direction, i.e., the operational amplifier A1 of the bandgap reference circuit 1 tries to reduce the electrical current inside the bandgap reference circuit 1. If, however, the start-up circuit 42 has already reached a start-up state at which it would like to turn off, it is evident that the bandgap reference circuit 1 may never reach its required operating point. In fact to reach this operating point it would require a sufficient electrical current to flow through the two diodes D1, D2, so that the operational amplifier A1 is driven beyond its own offset voltage. Only then will the automatic control work satisfactorily.
The turn-off point of the conventional start-up circuit 42 is hence relatively critical. In particular, for relatively low supply voltages and output voltages and relatively low temperatures, the conditions described above may be so unfavorable that it becomes impossible to design the start-up circuit 42 using sensible component values.